The speed and capacity of today's computers requires storage systems that are capable of almost instantaneously retrieving and storing voluminous amounts of information. The preferred medium for achieving this objective is rotating data storage devices that store the information on disk files in dynamic storage devices or rigid disk drives. Disk files consist of one or more rotatable disks on which data is written concentrically and at extremely high densities. Each one of the thin concentric circles where data is stored is called a track. When the data storage device is in operation, an extremely sensitive transducer reads and writes information to and from selected tracks.
The transducer is usually attached to an air bearing slider which glides on a cushion of air nanometers away from the surface of the rapidly spinning disk. The transducer and air bearing slider assembly is also known as a "ehead" or head assembly.
A head suspension, which in turn may be attached to and moved by an actuator arm, is the spring element that supports the head. Head suspension refers to either a suspension with a head attached or a suspension designed to hold a head. A suspension (or suspension assembly) usually includes a load beam element, having carefully loaded spring regions and rigid regions, and a gimbal spring to hold the head level over the surface of the disk.
The carefully balanced combination of a suspension, a head, and other suspension related elements is known as a head suspension assembly (HSA). A multi-disk system will consist of several disks rotating in parallel to each other, with heads positioned both over the top and over the bottom surface of each disk by HSAs and actuator arms that resemble those of a turntable. The whole assembly moves in and out very quickly over the disk to access information.
The average time required to get the head to a track is called average seek time (typically less than 10 milliseconds). The average time required for data to reach the head from any given point on the track as the disk turns is called latency time (typically less than 6 milliseconds). Average access time is the combination of the average seek and latency times. Long access times result in significant delays due to the large number of data transfers required by today's systems. Therefore average access time is a crucial factor in marketing and operating a disk file system.
The closer the head can fly to the surface of the disk, the more densely can information be packed on its surface. Today's disk drives strive to reach head clearances close to 100 nanometers=0.1 micrometers (a human hair is 100 micrometers thick). Greater data densities allow for greater storage and smaller size. But the head must not touch the disk ("crash"), as the friction caused by the high rotational speed of the disk may damage the surface of the disk, thus destroying the data stored on it, as well as the head itself.
Constantly maintaining the desired head clearance is not an easy task as, when measured in nanometers, the surface of the disk is not flat and the head has to glide level and in parallel to the disk's contours. To compound the problem, the suspension which supports the head experiences extreme stresses as the actuator arm moves it rapidly from one concentric circle of data to the next. A suspension must be extremely stiff and rigid to withstand the shear forces of stop and go movement with minimum deflection. Stiffness measures the property of a material to resist deflection by the inertial loads involved in accelerations and decelerations. The suspension must also resist vibration after movement, as this delays the precise positioning of the head required for reading and writing closely packed data. Motion vibrations and excessive momentum cause the head to "overshoot" the intended thin track of data and take unacceptably long times to settle, thus causing errors and increasing the average access time. As momentum is directly related to mass, a heavy (large mass) head suspension assembly will increase momentum and average access times. The suspension must also account for thermal expansion and surface vibration due to external forces.
Since the revolutions per minute (RPM) of the disk are constant, the velocities of the surface of the disk and of the air stream increase as the head moves away from the center of the disk. Therefore, as the suspension moves from the inside track to the higher linear velocity outside tracks of the disk, it must resist changes in overall elevation. To hold the head at a controlled height the suspension must balance the pressure over the head to compensate the variable opposite lift of the changing air stream on the slider.
While being moved, the suspension must not twist (torque), or one corner of the head will be too close and the other too far from the disk surface. Yet, when flying over a single track, the suspension must be compliant to some pitch about a first axis and some roll about a second axis orthogonal to the first, in order to adjust the flight of the head over the contours of the disk. Thus, a flexible gimbal area which allows the head to remain level relative to the surface of the disk, even while a rigid region of the load beam experiences changes in inclination due to elevation changes, is desired. Yet, construction of this area must be as efficient as possible. Forming processes that change the topology of the suspension's surface to achieve gimbal flexibility add complexity to the manufacturing process.
The suspension and the head also must be very light and have a low mass to reduce inertial momentum during each positioning movement. A large mass would result in sluggish head movement, overshoot problems, crashes, errors, and long access time. The farther this added mass is from the center of rotation of the HSA, the more the suspension acts as a lever that magnifies its effect. This is compounded in more complex systems that consist of many suspensions moving in unison. Small reductions in the mass of each suspension also permit significant reduced power to the actuator assemblies having multiple suspensions. This results in reduced power consumption and reduced heat buildup.
During normal reading and writing operations a multitude of electrical signals must travel back and forth from the head. These signals encode the bits (ones and zeroes) of information just retrieved by the transducer or the new information to be stored to the disk. Today's systems write and read millions of bits in a matter of seconds.
To assure reliable and efficient systems, data transmission must be fast, yet very accurate. Letters and words are recognized by their respective bit sequences. An error in the transmission of one bit of information will result in an unidentifiable or mistaken sequence. Wrong sequences translate to wrong letters and eventually to garbled information and instructions.
Conductors must carry the bits back and forth between the head and the IC. Two of the factors likely to alter the precision of electrical data are parasitic capacitances and series resistances in these conductors. Both capacitance and resistance increase in a directly proportional relationship to the length of the conductor relaying the electrical signal. The weaker the signal, the more susceptible it is to these influences, leading to transmission errors.
In disk drives, the most troublesome signal conduction path occurs between the head and the IC. To be properly decoded and amplified, signals sensed by a head transducer must be relayed to the processing and amplifying circuitry. But the read signals travelling this path are relatively weak. Therefore, they are very susceptible to electrical distortion due to the thermal noise of the conductor resistance and to the low pass resonant circuit formed from the parasitic capacitance of the conductors and the inductance of the transducer coil. Even the seemingly short path between the head and circuitry located past the actuator arm may contain enough parasitic capacitance and resistance to significantly distort the signals and limit the data rate.
A solution to this problem is to shorten the distance between the head and the integrated circuit containing the processing and amplifying circuitry (commonly called the read/write chip). The shorter the conductor path the better the electrical purity of the signal. Some disk drive systems place an integrated circuit (IC) that performs the necessary functions on the rigid actuator arm.
The use of the actuator arm has the advantages of offering an ample stable rigid surface on which to place an IC and make all the necessary connections. The ICs required to drive the heads demand a large number of input, output, voltage, ground, and control signals. ICs are fragile, so it is preferable to place them on a rigid stable surface.
However, the path between the actuator arm and the head is still relatively long. Placing the IC even closer would further reduce the distorting influences. But this requires a flat rigid surface with enough room to place the IC and to route and connect the plurality of conductors (usually ten or more) required to relay signals to and from the IC. This surface must be flat and strong enough to shield the IC from twisting or bending to prevent breakage of the fragile ICs. To provide undesired thermal expansion, it also must provide a way to dissipate the heat generated by the electronic components in the IC.
The placement of the IC and the conductors must not affect the spring and load characteristics of the suspension. Added mass, especially near the head, greatly increases the inertial momentum and places great strain on the suspension. Also, not only do conductors require space, but they also have a stiffening effect on spring areas of the load beam and especially on the gimbal, which is usually the most flexible part of the suspension. Optimally, the electrical conductors and the IC must be securely attached in order to reduce movement and vibration which causes fluctuating input and output impedances. They must exhibit low profiles, in order to fit in today's compact disk drives, and must have reliable and easy-to-use connection points.